Rotary devices for use as pumps or motors usually suffer from certain basic design limitations, namely that they operate with maximum mechanical efficiency only over a relatively narrow range. That is to say, the output of a device is at a maximum over a relatively narrow range of rotational speeds. Above such range, the output of the device will drop significantly. Numerous proposals have been made to produce such rotary devices having a wider efficient working range, the usual idea behind such attempts being to employ a rotary type device, which provides positive displacement of the fluid material. However, the majority of positive displacement rotary devices suffer from other disadvantages. Some of them are excessively complex, resulting in great expense and high frequency of repair. Others suffer from problems of sealing working surfaces, and in others considerable wear is caused by friction. Still others suffer from difficulties in attempting to balance eccentric forces. Valving and porting of such devices is also a common problem.
In rotary devices of the gear type a common problem is the trapping and pressurization of liquid between the gears, resulting in noisy operation and low mechanical efficiency, particularly at high rotative speeds. Existing methods to solve this problem in gear-type devices are complex and expensive.
In internal gear-type rotary devices (commonly known as "gerotors"), there is an inner and an outer rotor. Teeth on the internal surface of the outer rotor are adapted to mesh with teeth on the inner rotor. Adjacent teeth on the same rotor define recesses therebetween. In known gerotors, there is usually one tooth more on the outer rotor than on the inner rotor. Both rotors rotate. However, because of the different number of teeth on each rotor, one rotor rotates faster than the other, and thus there is relative rotation between the two rotors. Each tooth translates from one recess into another adjacent recess on the other rotor. Fluid may be trapped between a tooth and the bottom of a recess.
It will of course be readily appreciated that the advantages obtained by providing an efficient rotary device using a positive displacement principle are very great. Thus, for example, such a device can theoretically be used both for the relatively low pressure, high flow rate applications, and may, with various engineering changes, be used for the pumping of liquids at high shaft speeds.
As mentioned, numerous attempts have been made to design rotary devices to take advantage of such wide-ranging applications. Some of such attempts have depended on a central rotor with movable vanes rotating in a chamber. Others have employed two rotors rotating in opposite directions with interlocking vanes. Still others have attempted to solve the problems by using eccentrically-shaped rotors rotating in a specially shaped chamber. However, all of these proposals suffer from one or other of the disadvantages noted above.
Other devices, although effective, are costly to machine and require maintenance of precise tolerances.